I believe my GPU video card may be overheating, which is odd because it has a built in fan. My hardware monitor has it at about 60c-65c when the crash occurs. I have a gefore 7800GS (AGP) that is only crashing when I play one game: CS:SOURCE! Doesn't crash during war3,bf2,WoW; only CS:S. It will crash me to my desktop with no errior message, it will just exit the game.

yes, maybe its your PSU not supplying enough power? mine does that in lots of games or used to until i upgraded my PSU. When games reach high amounts of graphical detail, your GPU has to work extra hard to deliver them. However without the right power supply, your GPU lacks juice and cant handle it. So it has to quit from the strain resulting in quitting to desktop. Try turning your graphics down a bit also. Just because you dont lag on high graphics does not mean your GPU isn't struggling.